2015/09/10

Introduction

MightyWatt electronic load can be used not only to test how a battery is discharged but it can also be used to charge the battery. In conjunction with a fixed-voltage power supply like a wall wart, MightyWatt can be turned into a USB programmable Li-Ion charger with CC/CV mode.

Typically, a Li-Ion battery is first charged by a constant current. When a certain voltage is reached, for example 4.2 V, the charger then keeps the voltage constant until the current drops below some pre-defined threshold. Then the charging process is considered finished.

You can use the MightyWatt in conjunction with a cheap fixed-voltage power supply to create such charger. By connecting MightyWatt in series with the battery and power supply, it will be possible to control the current (and voltage) while measuring voltage at the battery terminals. This is possible thanks to the differential voltmeter whose inputs can be at different potential than the power terminals.

Setting it up

First, be sure that you have a firmware version at least 2.5.1 and Windows control program 1.2.2.0 or higher (get it here).

So, how to do it? First, make the connection (Figure 1): Connect the positive terminal of the battery to the positive terminal of your power supply. Connect the negative terminal of the battery to the positive power terminal (PWR+) of the MightyWatt. The negative power terminal (PWR-) is then connected to the power supply ground. This signal ground will be also the earth ground because the PWR- is connected to earth via USB.

Now, connect the positive sense terminal (SENS+) to the positive terminal of the battery and the negative sense terminal (SENS-) to the negative terminal of the battery. In this way, you will be able to measure the voltage of the battery while controlling the charging current.

Let's set the control software correctly now (Figure 2). In this example, I will assume that the battery should be charged with 1 Amp to 4.2 V and then kept at 4.2 V until the current drops to 50 mA.

Safety first: Set up the watchdog. If anything goes wrong, the load will disconnect the battery. I have set the watchdog to stop if current exceeds 1.5 A.

Set the remote (4-wire) voltage sensing.

Define the constant current part of the charging process. I put 1 A for 3 hours (the time limit won't be reached so put something long here). Then define the skip conditon. That means this part of the charging will be terminated when the condition is met. I chose that exceeding 4.2 V will make the program go to the next step.

Define the constant voltage part. It is important to set "constant inverted phase voltage", I will talk about it later. In my example, I will keep 4.2 V for 3 hours (it will finish sooner so once again, put some long time here) and terminate this part when the current drops below 50 mA.

If you wish, set up logging and/or save the program items for future use.

Hit the Start button, you're ready to go!

Figure 2: Setting of a CC/CV mode for battery charger. Watchdog is enabled for safety.

What is the "constant inverted phase voltage" all about?

The second part of the charging process requires the voltage to be kept constant. However, it is not possible to directly use the hardware implemented constant voltage because it has an incorrect phase for this application. Normally, the constant voltage mode assumes that by increasing the current, the voltage will drop. This is normal behaviour of most sources. But look at this setup: increasing current to the battery will actually increase its voltage. It is the difference of power supply voltage and battery voltage that is in fact decreasing. But we want to measure (and control) the battery voltage, not the power supply that is actually loaded.

So the feedback has to be modified to a system where increasing the current will increase the voltage. In other words, the transistor in MightyWatt must open when the voltage is too low and close when it is too high. This is not implemented in hardware so the firmware must be called to help. In the mode named "constant inverted phase voltage" the firmware (Arduino sketch) will internally switch the load to constant current mode and then increase it when the voltage is lower than the set voltage or decrease the current when the voltage is higher than the set value.

The bottom line

With the new firmware and software, you can turn your MightyWatt (+ cheap power supply) into a great USB programmable Li-Ion battery charger! Use the special "inverted phase voltage" mode to keep the voltage constant in this setup. If you don't have the latest firmware and software, download it from the resource page. Don't forget to copy your calibration values.